ORIGINAL ARTICLE
Development and characterization of polyacrylonitrile
(PAN) based carbon hollow fiber membrane
1
2
Syed Mohd Saufi, and Ahmad Fauzi Ismail
Abstract
Saufi, S.M. and Ismail, A.F.
Development and characterization of polyacrylonitrile (PAN)
based carbon hollow fiber membrane
Songklanakarin J. Sci. Technol., 2002, 24(Suppl.) : 843-854
This paper reports the development and characterization of polyacrylonitrile (PAN) based carbon
hollow fiber membrane. Nitrogen was used as an inert gas during pyrolysis of the PAN hollow fiber memo
brane into carbon membrane. PAN membranes were pyrolyzed at temperature ranging from 500 C to
o
800 C for 30 minutes of thermal soak time. Scanning Electron Microscope (SEM), Fourier Transform
Infrared Spectroscopy (FTIR) and gas sorption analysis were applied to characterize the PAN based carbon
membrane. Pyrolysis temperature was found to significantly change the structure and properties of carbon
membrane. FTIR results concluded that the carbon yield still could be increased by pyrolyzing PAN memo
branes at temperature higher than 800 C since the existence of other functional group instead of CH group.
o
Gas adsorption analysis showed that the average pore diameter increased up to 800 C.
Key words : carbon membrane, hollow fiber, gas separation, polyacrylonitrile
1
2
M.Sc. (Gas Engineering), Lecturer, Ph.D. (Chemical Engineering), Prof., Membrane Research Unit,
Faculty of Chemical Engineering and Natural Resources Engineering, Universiti Teknologi Malaysia, 81310
Skudai, Johor Bahru, Malaysia
Corresponding e-mail : afauzi@utm.my
Received, 9 January 2003
Accepted, 10 June 2003
Songklanakarin J. Sci. Technol.
Vol. 24 (Suppl.) 2002 : Membrane Sci. &Tech.
844
Membrane technology is becoming more
useful for separation of gas mixture and offers
great advantages in its operations (Robeson,
1999). Polymeric membranes had been successfully used for this purpose. However, their permeability-selectivity combination is still not up to
the industries satisfaction and their application
is limited especially related to severe environment such as higher temperature and corrosive
operation. At present, carbon molecular sieves
membrane (CMSM) has been identified as a
solution to this problem (Koros and Mahajan,
2000). It offers a balance on permeability-selectivity trade off and the separation can be maintained in the environment that was prohibited by
polymeric membrane previously. Ismail and David
thoroughly reviewed the advantages of carbon
membrane and the potential application for gas
separation (Ismail and David, 2001).
The interest in developing carbon membrane only grow after Koresh and Soffer (Koresh
and Soffer, 1987; 1986; 1983) had successfully
prepared apparently crack-free carbon hollow
fiber membranes by carbonizing cellulose hollow
fibers. CMSM contains narrow constrictions at
the entrance of the micropores that approach the
molecular dimensions of gaseous species. The
constrictions allow the passage of the smaller
species and restrict the larger ones as ruled in
a molecular sieving mechanism (Fuertes and
Centeno, 1999). It is believed that CMSM has a
pore size in the range of 3Å to 6Å (Fuertes and
Centeno, 1998) that enhance the discrimination
between gas molecules of different sizes.
CMSM can be produced by pyrolyzing a
polymeric membrane precursor in a controlled
condition and atmosphere. Lately, numerous precursors have been used to form carbon membrane such as polyimides and derivatives, polyacrylonitrile, phenolic resin, poly (vinylidene
chloride), poly (furfuryl alcohol), cellulose, phenol formaldehyde, polyetherimide and polypyrrolone. Many researchers used polyimides as
their precursor for producing carbon membrane.
Jones and Koros (Jones and Koros, 1994) reported that the best carbon membrane, in terms
Polyacrylonitrile (PAN)
Saufi, S.M. and Ismail, A.F.
of both separation and mechanical properties,
were produced from the pyrolysis of aromatic
polyimides.
However, polyimides are commercially expensive materials and some can be obtained in
laboratory scale only (Centeno and Fuertes, 1999;
Fuertes et al, 1999). Therefore, in order to reduce
the cost and time for carbon membrane fabrication, other alternative polymer must be considered. Therefore, the objective of this study is to
report the development of PAN based carbon
hollow fiber membrane. We have selected PAN
as a precursor for carbon membrane based on
many reasons as will be discussed in the following section. PAN is spun using dry-wet spinning
process to form hollow fiber membrane. Inert gas
pyrolysis system is used to pyrolyze the PAN
membrane into carbon hollow fiber membrane.
Experimental
Polyacrylonitrile (PAN)
PAN is one of the versatile polymers that
is widely used for making membranes due to its
good solvent resistance property. It has been
used as a substrate for nanofiltration (NF) and
reverse osmosis (RO) (Kim et al, 2002). The
thermosetting characteristic offered by PAN
makes it suitable as a carbon membrane precursor. It usually does not liquefy or soften during
any stage of pyrolysis and preserves its morphology upon the pyrolysis. The general molecular
structure of PAN is shown in Figure 1.
In carbon fiber manufacturing, PAN is
recognized as the most important and promising
precursor for carbon fiber. It dominates nearly
90% of all worldwide sales (Gupta and Harrison,
Figure 1. Molecular structure of polyacrylonitrile
(PAN)
Songklanakarin J. Sci. Technol.
Vol. 24 (Suppl.) 2002 : Membrane Sci. &Tech.
1999). There are numerous PAN fibers advantages including a high degree of molecular
orientation, higher melting point (PAN fiber
tends to decompose before its melting point, Tm
o
of 317-330 C) and a greater yield of the carbon
fiber (Donnet and Bansal, 1984). PAN fibers
form a thermally stable, highly oriented molecular structure when subjected to a low temperature heat treatment, which is not significantly
disrupted during the carbonization treatment at
higher temperatures, meaning that the resulting
carbon fibers have good mechanical properties
(Donnet and Bansal, 1984).
On the other hand, it is possible to blend
the PAN with other polymers to alter the final
pore size distribution of carbon membrane.
Linkov et al., (Linkov et al, 1994a) prepared
PAN hollow fiber membrane by varying the viscosity of the precursor solution by blending it
with methyl methacrylate. They also applied
phase inversion by casting PAN with poly (ethylene glycol) (PEG) and poly (vinylpyrrolidone)
(PVP) in order to synthesize membranes with 50400 nm pore sizes. The pore size distribution was
very narrow in the case of the PAN-PVP precursor
(Linkov, 1994b).
One of the main problems of unsupported
carbon membrane (i.e. hollow fiber) is brittleness
of the carbon structure. We believe that by the
application of PAN precursors, which is widely
used in the production of high strength carbon
fiber, this problem can be minimized. However, in
this case, first task is to develop PAN-based carbon membrane. Furthermore, some improvement
and optimization must be done in the next stage
in order to achieve high performance PAN-based
carbon membrane.
Preparation of carbon membrane
Dry/Wet spinning process
A binary polymer solution consisting of
150g PAN (Aldrich 18131-5) and 850g dimethylformamide (DMF) was prepared for making
1000g of spinning solution. Dry/wet-spinning process was applied in preparing asymmetric PAN
hollow fiber membranes precursor. Spinning so-
845
Polyacrylonitrile (PAN)
Saufi, S.M. and Ismail, A.F.
lution and bore fluids were extruded simultaneously through a spinneret to form a nascent
hollow fiber at an ambient temperature. The
internal surface of hollow fiber was contacted
with a bore fluid and experienced wet phase
separation in order to form a circular hollow
lumen. Water was chosen as bore fluid with flow
rate of lml/min (Pesek and Koros, 1994; Clausi
and Koros, 2000), which was controlled by highpressure syringe pump.
The fiber was then directed to a force convective chamber, which provides a controlled
force convective environment for inducing dry
phase separation through an air gap. The inert gas
nitrogen was used to allow the fiber skin formation. The hollow fiber was then immersed into
nonsolvent (water) coagulation bath for wet
separation and then the washing treatment bath.
The coagulation bath temperature was controlled
o
at 14 C by a refrigeration/heating unit to ensure
rapid solidification, while the washing bath was
kept at ambient temperature.
The resulting hollow fibers had an outer
diameter of 600µm with a 300µm inner diameter. The fully formed PAN hollow fiber was
subjected to solvent exchange in methanol for 2
days and hexane for another 2 days before being
dry in ambient atmosphere.
Inert gas pyrolysis
Inert gas nitrogen was set up for pyrolysis
of PAN hollow fiber membrane as shown in
Figure 2. PAN hollow fiber bundles were inserted into a quartz tube and wrapped with stainless steel wire type 304 (outside diameter
3 mm) at both ends. Then, the quartz tube was
inserted into Carbolite wire wound tube furnace
(Model CTF 12/65/550) that can be set to a maxio
mum temperature of 1200 C. The pyrex socket
was connected at the front of quartz tube to
channel the nitrogen gas during pyrolysis and
the other side of quartz tube was located in a fume
cupboard to purge the volatile gas evolved. All
the connection were properly tighten to prevent
air from entering the quartz tube which could
interrupt the inert gas pyrolysis process.
Songklanakarin J. Sci. Technol.
Vol. 24 (Suppl.) 2002 : Membrane Sci. &Tech.
846
Polyacrylonitrile (PAN)
Saufi, S.M. and Ismail, A.F.
Figure 2. Nitrogen gas pyrolysis system
The PAN membrane was subjected to thero
mostabilization process in air at 250 C for 30
o
min at heating rate of 5 C/min before the pyrolysis was conducted. Thermostabilization is necessary in order to cross-link the PAN chains and
to prepare a structure that can withstand high
temperature process. This can also ensure both the
molecules and the fibrillar orientation will not be
eliminated during the final heat treatment process.
Before the pyrolysis process started, the
inert gas needed to be purged into the pyrolysis
system to remove the unwanted air or oxygen.
This was to prevent the oxidation from occurring
during high temperature pyrolysis process. Then,
the precursor was heated to a required pyrolysis
o
temperature in the range of 500-800 C and maintained at that temperature for 30 min by setting
up the temperature control systems. The heating
o
rate was set at 3 C/min and nitrogen gas flow rate
3
was maintained at 200 cm /min. The resulting
carbon membrane was cooled down to ambient
temperature in an inert gas atmosphere.
Characterization of carbon membrane
Scanning electron microscopy (SEM)
Scanning Electron Microscopy (SEM) has
been used to investigate the resultant membrane
morphology. Images of fiber surface, skin layer
structure and cross sections of membrane pre-
pared under different carbonization condition can
be viewed clearly. Before passing through SEM,
the membrane samples had to go through the gold
coating process. After that, the samples were
imaged and photographed by employing a scanning electron microscope (SEMEDAX; XL40:
PW6822/10) with potentials of 10kV in achieving
magnification ranging from 500x to 5000x.
Fourier transform infrared spectroscopy
(FTIR)
Fourier Transform Infrared Spectroscopy
(FTIR) is a very useful tool to detect the existence of functional groups in a membrane. The
FTIR results display changes of functional groups
and elements in the membranes when they are
heated from room temperature to pyrolysis temperature.
Gas sorption analysis
Nova 1000 Gas Sorption Analyzer, Quantachrome Corporation was used to determine the
average pore size and micropore volume of carbon membrane. The sample was pretreated in a
o
vacuum at 400 C for about 2 hours. The heated
sample cell was immersed into liquid nitrogen to
maintain the temperature at 77 K before testing.
Nitrogen gas was used as an analysis gas.
Songklanakarin J. Sci. Technol.
Vol. 24 (Suppl.) 2002 : Membrane Sci. &Tech.
Results and Discussion
Effects of pyrolysis temperature on the morphology of PAN-based carbon membrane
Changing the high temperature treatment
parameter can vary the permeation characteristics
of a carbon membrane. This is in contrast to
polymeric membranes, where such variations
frequently involve the synthesis of a new membrane material (Koresh and Soffer, 1986). This
unique characteristic is one of the advantages of
carbon membrane for gas separation.
From SEM image in Figure 3, the structure
of PAN carbon membrane changes greatly between the inner and outside diameter of the fiber
(i.e. middle region) compared to PAN precursor.
For the PAN-based carbon membrane, the outer
selective skin is clearly separated from the inner
wall by the larger macrovoid between it. Meanwhile for PAN membrane, the pores from the inner
diameter are likely connected to the outer diameter pores. This structure could be a reason for the
imbalance of permeability-selectivity in polymeric
membrane, where the higher selectivity give the
lower permeability to gas separation. On the other
hand, the larger macrovoid presented in carbon
membrane contributes to a higher permeability
without interrupting the higher selectivity. Higher
(a) PAN Membrane
847
Polyacrylonitrile (PAN)
Saufi, S.M. and Ismail, A.F.
selectivity is determined by the pore constriction
at the outer surface of the carbon fiber. Therefore,
carbon materials not only have the ability to perform molecular sieving but also may allow a
considerably higher flux of the penetrant through
the material (Steel and Koros, 2003).
Pyrolysis temperature will greatly change
the morphology and structure of PAN membrane.
For PAN precursor, the membrane must be pyroo
lyzed at a minimum temperature of 400 C (David,
2001). Figure 4 shows the variations of the
PAN-based carbon membrane structure with the
o
pyro-lysis temperature ranging from 500-800 C.
The entire SEM images were at 800X magnification. It can be observed that the amount of the
outer pores skin was increased with the increasing
pyrolysis temperature. During the pyrolysis, byproducts of different volatilities are evolved and
generate pores through the polymeric matrix
(Steel, 2000). Therefore, as the pyrolysis temperature increases the amount of by-products also
increases which enhances the pore formation.
However, at relatively higher pyrolysis temperature, the pore structure becomes tighter which
gives an increase of gas selectivity (Geiszler and
Koros, 1996). Densification of the porous carbon
matrix occurs at high pyrolysis temperature,
which can be identified by an increase of shrink-
(b) PAN Based Carbon Membrane
Figure 3. Cross section of PAN membrane and PAN based carbon membrane
Songklanakarin J. Sci. Technol.
Vol. 24 (Suppl.) 2002 : Membrane Sci. &Tech.
Polyacrylonitrile (PAN)
Saufi, S.M. and Ismail, A.F.
848
Figure 4. Carbon membrane at a different pyrolysis temperature
age factor as shown in Table 1.
Effects of pyrolysis temperature on the functional groups evolution during pyrolysis
Generally, carbon membrane is known by
its amorphous porous structure created by the
evolution of gases generated during pyrolysis of
the polymeric precursor. Though the membrane is
amorphous in nature, one still finds carbon membrane subdomains, where the structure of the
polymeric precursor is still recognizable. This
subdomain structure determines, in part, the differences that are found in the performance of
carbon membrane derived from various polymeric precursors. The extent of this subdomain
structure also depends on the pyrolysis conditions
(Sedigh et al, 1999).
Table 1. Percentage of shrinkage based on the
variations of inside diameter of the fiber.
Pyrolysis
temperature
Inside
µm)
diameter (µ
% Shrinkage
PAN membrane
o
CM 500 C
o
CM 600 C
o
CM 700 C
o
CM 800 C
300
249
216
202
204
17.0%
28.0%
32.7%
32.0%
The mechanism of carbon membrane formation from a PAN is a complex process and
various reaction schemes have been proposed to
occur especially by researchers involved in carbon
fiber production. During oxidative stabilization
Polyacrylonitrile (PAN)
Saufi, S.M. and Ismail, A.F.
Songklanakarin J. Sci. Technol.
Vol. 24 (Suppl.) 2002 : Membrane Sci. &Tech.
849
Table 2. Summary of FTIR results for PAN membranes pyrolyzed at different temperature
Pyrolysis
temperature
o
500 C
Figure 5. Structure of ladder polymer from PAN
process, cyclization and oxidation taking place to
form a ladder polymer as shown in Figure 5
(David and Ismail, 2003). This ladder structure
is thermally stable and ready to withstand high
temperature of pyrolysis process.
Pyrolysis of PAN membrane involves
many process such as chain scission, chain breakdown, by-products evolution or cross linking depending on heat treatment history. The principal
scission products releases from PAN precursor in
o
the temperature ranging from 400 to 1000 C are
hydrogen cyanide (HCN), ammonia (NH3) and
nitrogen (N2). Depending on the extent of pre-oxidation, various amounts of water (H2O), carbon
monoxide (CO) and carbon dioxide (CO2) were
also formed. In addition, small quantities of hydrogen (H2) and methane (CH4) were released
during the carbonization process (Lewin and
Preston, 1983). Figure 6 shows the proposed
mechanism of the stabilization chemistry and
carbonization of PAN-based carbon fiber.
Figure 7 and Figure 8 show the spectrum of
pyrolyzed PAN membrane at different pyrolysis
temperatures. Meanwhile Table 2 gives a summary of FTIR results for PAN membrane pyrolyzed
using nitrogen pyrolysis system. The original
PAN molecule consists of functional groups such
as methyl (CH3) and nitrile (C≡N). During oxidation, numerous new transition structures were
formed, such as ketones, aldehydes, carboxilyc
acid etc. as in Figure 6. After carbonization, all
of these transition compounds were expected to
evolve as volatilities and only carbon and hydrogen atoms remain. However, 100% conversion of
polymer to carbon is not achieved due to the
o
600 C
o
700 C
o
800 C
Functional
group
Frequency
-1
(cm )
C≡N
C-N
C=C
2216.42
1265.90
1595.42
C≡N
C=N
C-N
C=C
N-H
C=O (Aldehydes)
C=O (Ketones)
2224.05
1649.36
1265.77
1539.65
3445.29
1739.53
1701.79
C=N
C-N
C=C
N-H
C=O (Ketones)
C-H
1653.23
1021.84
1540.69
3442.13
1701.01
2924.63
C=N
C-N
C=C
N-H
C-H
1649.58
1021.39
1541.42
3442.97
2923.64
existence of other compounds.
The C=C groups exist in all pyrolyzed
PAN membrane. This group is formed by aromatization process that occurred during thermostabilization of PAN membrane. The nitrile groups
(C≡N) disappear when PAN membrane is pyrolyzed
o
at temperature of 700 C. Aldehydes and ketones
can be detected at pyrolysis temperature of
o
600 C and aldehydes disappeared when reaching
o
o
700 C. A ketone group disappeared at 800 C
spectrum but nitrogen atom can still be found at
this spectrum. It can be concluded that pyrolysis
o
temperature at 800 C seems not enough to remove all non-carbons atoms (i.e. N, O, etc) in
PAN membrane because of the existence of other
functional group instead of CH group as detected
by FTIR spectrum.
Songklanakarin J. Sci. Technol.
Vol. 24 (Suppl.) 2002 : Membrane Sci. &Tech.
850
Polyacrylonitrile (PAN)
Saufi, S.M. and Ismail, A.F.
Figure 6. Proposed mechanism of the stabilization chemistry and carbonization of
PAN based carbon fiber (Fitzer et al, 1986)
Effects of pyrolysis temperature on the porous
structure of carbon membrane
The mechanism of gas permeation and uptake through porous solids is closely related to
internal surface area, dimension of the pores and
surface properties of the solid, rather than to the
bulk properties of the solid as in the case with
polymers (Koresh and Soffer, 1987). Therefore, it
is important to study and analyze the porous
structure in carbon membrane. The porous structure in carbon membrane contains pore or apertures that approach the molecular dimension of
the diffusing gas molecules (Jones and Koros,
1994; Lafyatis et al, 1991). According to the
IUPAC, three different types of pore have been
defined; macropores that are larger than 50 nm;
mesopores, between 2 and 50 nm in size; and
finally, micropores, that are smaller than 2 nm.
Generally, the pore size of carbon membrane is non-homogeneous, comprised of relatively wide openings with a few constrictions
(Koresh and Soffer, 1980). The pores are varying
in size and dimensions depending on the morphology of the organic precursor and the chemistry of pyrolysis.
The idealized structure of
carbon material is shown in Figure 9. The pore
mouth “d”, often referred to as an ultramicropore
(<10Å) (Suda and Haraya, 1997; Centeno and
Fuertes, 1999), which allows molecular sieving of
the penetrate molecules. The larger micropores,
“D” of the material (6-20Å) may allow for diffusion of gas molecules through the carbon material
(Steel, 2000).
The extent and nature of the porous structure and the surface area will be largely dependent
on the nature and morphology of the precursor
Songklanakarin J. Sci. Technol.
Vol. 24 (Suppl.) 2002 : Membrane Sci. &Tech.
851
Polyacrylonitrile (PAN)
Saufi, S.M. and Ismail, A.F.
o
(a) 500 C
o
(b) 600 C
o
o
Figure 7. FTIR spectrum of pyrolyzed PAN membrane at (a) 500 C (b) 600 C
polymer and the history of its heat treatment
(Donnet and Bansal, 1984). The imperfect stacking between the microfibrils of graphite basal
plane in carbon fiber gives rise to empty spaces
between the microfibrils that form pores or
voids. Generally, low temperature carbon fibers
have small but numerous pores that are distributed throughout the carbonized material (Donnet
and Bansal, 1984).
This is in agreement with the results
shown in Table 3. It can be observed that the
average pore diameter increases with increasing
o
the pyrolysis temperature up to 800 C due to
the expelling of the carbon atoms as carbon mon-
oxides (David, 2001). However, at a very high
temperature, pore size start to decrease and
eventually the pores will shrink and collapse
owing to progressive annealing (Suda and
Haraya, 1997; Centeno and Fuertes, 1999). The
BET surface area is increased as the pyrolysis
temperature reaches the maximum to a value of
2
103 m /g, and then decreases rapidly. This trend
also agrees with the results reported by Lee and
Tsai (Lee and Tsai, 2001).
Conclusion
Based on the results presented, it can be
Songklanakarin J. Sci. Technol.
Vol. 24 (Suppl.) 2002 : Membrane Sci. &Tech.
Polyacrylonitrile (PAN)
Saufi, S.M. and Ismail, A.F.
852
Table 3. Micropores properties of PAN membrane pyrolyzed at different pyrolysis
temperature.
Pyrolysis
temperature
o
500 C
o
600 C
o
700 C
o
800 C
Average pore
diameter (Å)
Micropore volume
(cc/g)
BET surface area
2
(m /g)
11.19
10.20
12.43
13.13
0.039
0.032
0.040
0.034
108.29
95.47
102.98
85.66
o
(a) 700 C
o
(b) 800 C
o
o
Figure 8. FTIR spectrum of pyrolyzed PAN membrane at (a) 700 C and (b) 800 C
Songklanakarin J. Sci. Technol.
Vol. 24 (Suppl.) 2002 : Membrane Sci. &Tech.
Figure 9. Idealized structure of carbon material
(Steel, 2000)
concluded that PAN membranes can be successfully pyrolyzed into carbon membrane using
nitrogen gas pyrolysis system. Pyrolysis temperature influences the resultant carbon membrane
by altering the structure and pore properties of the
membrane. FTIR results concluded that the carbon yield still could be increased by pyrolyzing
PAN membranes at temperatures higher than
o
800 C because of the existence of other functional group instead of CH group. Gas adsorption
analysis showed that the average pore diameter
o
increased up to 800 C. This result encourages
further research on higher pyrolysis temperature
since it has been hypothesized that the pore diameter will shrink at relatively higher temperature.
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